1. Field of the Invention
The present invention relates to a color electrophotographic apparatus which performs halftoning operations through use of halftone spots formed by a plurality of dots, a method of processing an image produced by the color photographic apparatus, and a recording medium having recorded thereon a program to be used for image processing. In this specification, the word xe2x80x9cdotxe2x80x9d implies xe2x80x9cpixelsxe2x80x9d defined as a unit cell in electrophotographic apparatuses.
The present application is based on Japanese Patent Application No. Hei. 11-28666, which is incorporated herein by reference.
2. Description of the Related Art
In an electrophotographic apparatus, such as a color printer or a color copier, a color image is reproduced by utilization of cyan toner, magenta toner, yellow toner, and black toner. Particularly, among color printers, some page printersxe2x80x94which forms a latent image on a photosensitive drum by utilization of a laser beam, develop the latent image by use of charged toner, and transfer an image formed from the thus developed toner onto transfer paperxe2x80x94can change an area to be exposed by the laser beam within a dot in various manners. Thus, even when the number of dots per unit area is small, those page printers can reproduce a color image with high resolution and high gradation.
In such a color electrophotographic apparatus, a dithering method has been widely utilized as a binary-coding method to be used for reproducing the halftone of a gray-scale image. According to the dithering method, by reference to conversion tables which are called dither matrices or threshold-value matrices and which define the correspondence between halftone data and image production data, a determination is made as to whether color spot is displayed in each dot or not. A dot is xe2x80x98ONxe2x80x99 when color spot is displayed and xe2x80x9cOFFxe2x80x9d when color spot is not displayed. Halftone spots are produced by one dot or some adjacent dots turning xe2x80x9cONxe2x80x9d, and halftones of the images are reproduced on the basis of the sizes of halftone spots.
Dots are arranged in the direction of primary scanning in which a laser beam is moved for scanning (hereinafter referred to simply as a xe2x80x9cprimary-scanning directionxe2x80x9d) and in the direction of secondary scanning in which transfer paper is fed (hereinafter referred to simply as a xe2x80x9csecondary-scanning directionxe2x80x9d). As some dots become xe2x80x9cONxe2x80x9d and thus form the xe2x80x9ccore of the growthxe2x80x9d of halftone spots. As the gray-scale level of the halftone data is increased further, the number of xe2x80x9cONxe2x80x9d dots is eventually increased, thus gradually enlarging the size of he halftone spots.
FIG. 1 shows the combination of the angle of a cyan screen, the angle of a magenta screen, the angle of a yellow screen, and the angle of black screen, which has conventionally been used in wide applications of industrial printing systems. As shown in the drawing, according to the conventional technique, the angles of four color screens are set; specifically, the angle of the yellow (Y) screen is set to 0xc2x0; the angle of the cyan (C) screen [or the angle of the magenta (M) screen] is set to 15xc2x0; the angle of the black (K) screen is set to 45xc2x0; and the angle of the magenta (M) screen [or the angle of the cyan (C) screen] is set to 75xc2x0.
It is also known that, if the screen angles of the halftone spots are shifted in order to prevent chromatic misregistration, a so-called moire pattern appears. It has empirically been acknowledged that a shift of angle of about 30xc2x0 between two color screens is optimal for increasing the spatial frequency of the moire pattern, to thereby render the moire pattern inconspicuous. Yellow is less noticeable to the human eye. Therefore, the other 3 color screens (C,M,K) are set shifted from each other by 30xc2x0. Further, the angle of the black screen, which is most noticeable to the human eye, is set to 45xc2x0, so as be most distant from a longitudinal angle of 0xc2x0 and a horizontal angle of 90xc2x0, which are easily recognized by the human eye. The angle of the cyan screen is set to 15xc2x0, and the angle of the magenta screen is set to 75xc2x0. The angle of the yellow screen is set to 0xc2x0. Although the yellow screen is set to the longitudinal direction or the horizontal direction that are most noticeable to the human eye, the yellow screen does not become greatly noticeable, because yellow is least noticeable to the human eye.
As mentioned above, the industrial printing system is designed so as to prevent a moire pattern by setting the magenta or cyan screen to an angle of 15xc2x0 or 75xc2x0 and rotating the color screens. Since the color screens are only rotated, exactly as they are, the pitch among halftone spots is maintained uniform throughout the 4 colors.
In an electrophotographic apparatus utilizing a laser beam, the pattern of dots, which can be developed by an engine for developing an actual image on the basis of image reproduction data, is limited to the direction of primary scanning in which a laser beam is actuated for scanning, as well as to the direction of secondary scanning in which paper is fed. Unlike the industrial printing system, the electrophotographic apparatus is incapable of rotating the color screens to arbitrary angles. Accordingly, in the electrophotographic apparatus, desired screen angles are achieved by shifting the positions of the dither matrices to be used for the dithering method in primary or secondary scanning direction, or by changing the data in the conversion table, as required.
FIG. 2 is an illustration for describing a conventional method of determining screen angles in dithering method. In this example, dither matrices 40, each measuring mxc3x97m, are shifted from one another so as to correspond to image data, thus achieving a screen angle xcex8; i.e., tan xcex8=b/a. In amore specific example shown in FIG. 2, dither matrices 40 are shifted such that in a horizontal row of dither matrices 40, each dither matrix 40 is vertically shifted from the preceding dither matrix 40 by a given amount, such that after four shifts the last dither in the row is vertically shifted by an amount corresponding to the height of one dither matrix 40. Therefore, we have tan xcex8=xc2xc. A dither matrix 42 designated by broken lines comprises a plurality of dither matrices 40. It is possible to determine the screen angle at an arbitrary value with higher degree of freedom by means of such a large dither matrix 42.
A screen angle of 15xc2x0 for magenta and a screen angle of 75xc2x0 for cyan, which are deemed to contributed to the best picture quality in the printing industry, are related to an irrational tangent (i.e., a tangent which is an irrational number). Angles related to the irrational tangent cannot be reproduced, so long as a limited number of dots arranged in both the direction of primary scanning and the direction of secondary scanning are utilized. For this reason, in the conventional electrophotographic apparatus, the magenta screen and the cyan screen are set to angles which are related to a rational tangent [tan xcex8=a/b (where xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d are integers), and xcex8=15xc2x0 and xcex8= 75xc2x0 are not related to the rational tangent] and close to an angle of 15xc2x0 and an angle of 75xc2x0.
Another conceivable approach toward selecting angles which are related to the rational tangent and close to 15xc2x0 and 75xc2x0 is to increase the size of the dither matrices 42. However, the number of dots per unit area which the engine can process is as small as, e.g., 600 dpi (dots per inch). If the size of the dither matrices is increased, halftone spot pitch increases and screen frequency is diminished. Further, an increase in the size of dither matrices also results in an increase in the number of corresponding required xcex3 tables. Such an increase in the number of xcex3 tables in turn involves an increase in the volume of a recording medium for recording the conversion tables.
Eventually, the cost of the electrophotographic apparatus is increased.
In a case where halftone spots are formed by utilization of the dots fixedly arranged in both the direction of primary scanning of the laser beam (i.e., the primary-scanning direction) and the direction of secondary scanning of the same (i.e., the secondary-scanning direction), the pitch between halftone spots among the color screens of different angles cannot be made uniform. Even in this respect, the electrophotographic apparatus encounters difficulty providing the same picture quality as that provided by the industrial printing system.
An object of the present invention is to provide an electrophotographic apparatus capable of achieving screen angles related to an irrational tangent, a method of processing an image produced by the electrophotographic apparatus, and a recording medium on which an image processing program is recorded, with regard to a color electrophotographic apparatus which reproduces an image by utilization of halftone spots formed from a plurality of dots.
Another object of the present invention is to provide an electrophotographic apparatus capable of making uniform pitches of halftone spots in a screen compatible with a plurality of colors, a method of processing an image produced by the electrophotographic apparatus, and a recording medium on which an image processing program is recorded, with regard to a color electrophotographic apparatus which reproduces an image by utilization of halftone spots formed from a plurality of dots.
Still another object of the present invention is to achieve the objects through use of a reduced number of conversion tables.
An image reproduction engine which causes toner to adhere to a development region of certain area located at a certain position within dots according to image reproduction data is utilized for image processing, wherein a halftone is expressed by means of halftone spots formed from a plurality of dots. The centroid of a halftone spot formed from a single dot or a plurality of adjacent dots can be shifted from the center of the dot to an arbitrary position, thus achieving desired screen angles or desired spot pitches. As a result, screen angles related to an irrational tangent can be realized, and the pitches of halftone spots of a plurality of color screens can also be made uniform.
In the case of an electrophotographic apparatus which radiates a laser beam onto a region of dots while being scanned in a given direction, the present invention enables the position and area where the laser beam is radiated to be controllably changed to an arbitrary position and area for each dot, by producing a laser drive pulse signal according to image reproduction data by means of pulse-width modulation (PWM), as required.
In order to reduce the volume of conversion tables which are provided within a controller of an electrophotographic apparatus or a driver of the host and which define the correspondence between halftone data and image reproduction data, the present invention utilizes an index-type conversion table. The conversion table comprises a plurality of xcex3 tables defining the correspondence between halftone data and image reproduction data, and a pattern matrix which includes reference data representing xcex3 tables to be referred to so as to correspond to a matrix including a plurality of dots. By means of such a configuration, some of reference data sets in the pattern matrix can be identical, and a single xcex3 table can be referred to by a plurality of dots within the pattern matrix.
To achieve the objects of the present invention, the present invention provides a color electrophotographic apparatus which reproduces an image by utilization of a plurality of color toners and by expressing halftone of each color through use of halftone spots formed from a plurality of dots, the apparatus comprising: a halftone processing section which is provided with halftone data for respective colors and which reproduces the image reproduction data corresponding to the dots on the basis of the halftone data by reference to a conversion table defining the correspondence between the halftone data prepared so as to correspond to the dots and image reproduction data; and an image reproduction engine which is provided with a drive signal corresponding to the image reproduction data and which causes the toners to adhere to a development region whose area and location correspond to the image reproduction data, within the dots, wherein the halftone processing section prepares the image reproduction data to be used for changing the angle of one color screen of the plurality of color screens to substantially an angle related to an irrational tangent.
Further, to achieve the objects, the present invention provides a color electrophotographic apparatus which reproduces an image by utilization of a plurality of color toners and by expressing halftone of each color through use of halftone spots formed from a plurality of dots, the apparatus comprising: a halftone processing section which is provided with halftone data for respective colors and which reproduces the image reproduction data corresponding to the dots on the basis of the halftone data by reference to a conversion table defining the correspondence between the halftone data prepared so as to correspond to the dots and image reproduction data, and an image reproduction engine which is provided with a drive signal corresponding to the image reproduction data and which causes the toners to adhere to a development region whose area and location correspond to the image reproduction data, within the dots, wherein the halftone processing section prepares the image reproduction data to be used for making the distances among the centers of the halftone spots of the plurality of colors substantially equal.
The present invention also provides a recording medium which reserves an image processing method for use with the foregoing electrophotographic apparatus and a program used for effecting image processing.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.